1. Field of Invention
This invention relates to a method and system for using optical phase conjugation in an optical communications network.
2. Description of Related Art
Fiber-optic communication networks are experiencing rapidly increasing deployment. Especially rapid is the growth of segments that carry multi-gigabit digital data on multiple wavelengths over a single fiber strand, which are known as wavelength division multiplexing (WDM). The wavelength channel density (i.e., the number of channels carried by one fiber) and the data rate carried on individual wavelengths continue to increase. Current systems employ 50 GHz channel spacing, but 25 GHz and 12.5 GHz channel spacings are expected to be utilized. Data rates of 10 Gbps are currently used, but 40 Gbps data rates are also expected. Both increased channel density and increased data rate contribute to increased crosstalk between channels.
Several linear and non-linear effects contribute to the deterioration of the signal and crosstalk. One linear effect that limits transmission length is chromatic dispersion, which causes signal bits to spread due the wavelength dependence of the index of refraction of the transmission fiber. Since a bit contains many wavelengths traveling at different speeds, the bit tends to distort as it travels along the transmission fiber resulting in inter-symbol interference and bit errors.
Non-linear effects also induce crosstalk and deteriorate signal quality. For passive optical fibers, the crosstalk mechanisms include cross-phase modulation, four-wave mixing, and Raman crosstalk. Further, active components such as fiber-based or semiconductor based optical amplifiers will add cross-gain modulation. These nonlinear crosstalk effects, arising due to the fiber Kerr non-linearity and due to the Raman effect, are additive to the overall interference level. The additive effect occurs in terms of each additional wavelength channel contributing a crosstalk component to the overall interference level. The additive effect also occurs in systems that have multiple optical spans with intermediate optical amplification, such that each span additively contributes a crosstalk component to the overall noise level. Accordingly, the additive non-linear effects can significantly impact performance of WDM transmission systems, particularly those over long distances, having multiple spans, each including amplifiers.
The non-linear effects described above, specifically self-phase-modulation and cross-phase-modulation are coupled to dispersion compensation. Specifically, the non-linear phase imparted to an information bearing channel from its own intensity modulation (i.e. self-phase-modulation or SPM) or due to the intensity modulation of its neighbors (cross-phase-modulation or XPM) gets converted to intensity noise through dispersion. For example, the amount of intensity noise generated due to SPM and XPM at a direct detection receiver (which is insensitive to optical phase noise), depends not only on the amount of phase noise generated through the non-linear process, but also the extent to which this phase noise gets converted to intensity noise from uncompensated dispersion.
A simple and conventional way of mitigating the conversion to intensity noise would be to perfectly compensate the dispersion experienced by the channel. In a multi-span, multi-channel WDM system, this amounts to bringing the residual dispersion to zero at the end of each span for all channels.
While this technique of compensating dispersion does negate XPM and SPM effects in amplitude-modulated channels, it can cause resonance effects since all signals are compressed back to their start positions and are also all aligned in time (i.e., there is no time skew between signals). To avoid resonance effects, the dispersion at the end of each span is not allowed to be zero, but is kept at some small positive or negative value. Further, phase-modulated channels that use phase-sensitive receivers may require a different optimization of dispersion along the fiber link.
A second complication of this solution that arises in multi-channel systems is the inability to perfectly match the dispersion slopes of the transmission fiber and dispersion compensating fiber. This results in different channels experiencing different levels of residual dispersion at the end of each span.
A third problem with mitigating XPM and SPM effects with perfect per-span dispersion compensation is the distributed nature of SPM and XPM generation. The non-linear phase shift from SPM and XPM is generated most strongly in the sections of the fiber where the optical power is highest and amplitude variations are most rapid. For amplitude-modulated channels, this section is typically in the first few kilometers of the transmission fiber following an optical amplifier. After the first few kilometers, the signals diverge (e.g., walk-off) away from each other which reduces XPM and/or reduce in intensity due to the fiber attenuation which reduces both XPM and SPM. Since the non-linear phase shift is generated over several kilometers, the compensation has to also occur over a similar distance.
A fourth problem with mitigating SPM and XPM effects are non-overlapping dispersion optimization points for SPM compensation and XPM compensation. In some situations, small amounts of SPM are used to actually enhance system performance. For all these reasons, as transmission distances increase, it becomes more difficult to mitigate XPM and SPM generated noise by simple dispersion compensation or dispersion mapping.
Existing solutions to counteract linear effects and non-linear effects and extend system reach include optimal dispersion mapping, use of slope compensating DCF fibers, use of Raman amplification, use of Forward Error Correction (FEC), and use of dynamic gain equalizers to flatten the spectrum. Each solution on its own and in conjunction with others can extend the reach of WDM systems. However, all solutions to counteract linear effects and non-linear effects either reduce the launch power required to maintain a required signal to noise ratio for detection (Raman amplification and FEC), or make the non-linear effects more symmetric across channels so some channels are not overly penalized (Dynamic Gain Equalizers), or reduce the conversion of non-linear phase noise to intensity noise (slope compensating DCFs and dispersion mapping).
Even after incorporating all the above solutions, substantial amounts of non-linear effects are still present in WDM systems, especially for systems having closely spaced channels, long transmission distance and/or higher data rates. None of the known techniques offers a solution that neutralizes non-linear effects that are present after all of the commonly used solutions are used.
Optical Phase Conjugators (OPCs) provide a means for compensating for the non-linear effects. Optical phase conjugation works on the principle of spectrum inversion. Basically, as an optical signal travels through an optical fiber it experiences optical phase shifts introduced both by itself and by adjacent optical channels. In the spectral domain, these non-linear effects change the frequency content of the signal. Such phase shifts and frequency components are added with signs determined by the intensity edge slope. If such a signal passes through a device (i.e., a phase conjugator) where its optical spectrum is inverted, (that is made into a mirror image of the input), then propagation through the remaining portion of the optical fiber tends to unravel the non-linear effects impressed on the signal prior to passing through the phase conjugator. If the first and second portion of the optical fiber (the first portion being before the conjugator and the second portion after the conjugator) are roughly equal in length, dispersion and optical power, complete cancellation of the non-linear effects can be achieved in theory.
Optical phase conjugation can also be used to cancel dispersive effects in optical fiber. Early applications of optical phase conjugators were for compensating linear dispersion. The early work considered only the linear dispersive signal distortion, which could be compensated by positioning the OPC in the center of the span. Subsequent applications included compensating intra-channel distortion, such as SPM, induced by Kerr effect in the fiber, by positioning the OPC in the center of the span. Such simultaneous compensation of chromatic dispersion and non-linear effects (e.g., SPM) places simultaneous constraints on the approximate equality of both transmission fiber dispersion and accumulated nonlinear phase shift on the opposite sides of the span.